Add background to embeds to solve dark mode issues

[pikkuruusu]

Some embeds are completely hidden by dark mode in app-article. We could probably solve this with css by just adding a white background to iframes. If there are other cases coming up we'd need to solve them separately.

Examples: https://app-article.ksfmedia.fi/index.html?uuid=ed5c62b1-1061-46ce-b227-8760ce4d90fe&paper=hbl&mode=dark
https://app-article.ksfmedia.fi/?uuid=4ac3f56d-c368-453f-a2ca-ef49d9f61762&paper=hbl&mode=dark